FIG. 1 illustrates a power supply control device 100 of a double conversion system. The power supply control device 100 includes a DC-DC converter 104 and an LDO (low drop regulator) 110. The respective DC-DC converter 104 and the LDO 110 are controlled independently. The DC-DC converter 104 is controlled by a DC-DC converter control circuit 111. The LDO 110 is controlled by an LDO control circuit 112. An output voltage Vdc is output by a boost control of the boost DC-DC converter 104 responsive to an input voltage Vin. An output voltage Vo is output by a reduction control of the LDO 110 responsive to the output voltage Vdc. The output voltage Vo is supplied to a load 108.
As shown in FIG. 2, the output voltage Vo is 0 V at a time t101 during standby. A load current does not flow through the load 108 during standby. The output voltage Vo gradually rises during a start up period from the time t101 to a time t102. This prevents an inrush current from flowing through the load 108.
As further shown in FIG. 2, a value of the output voltage Vdc is set to be greater than that of the output voltage Vo by a predetermined voltage or more because the LDO 110 in FIG. 1 is a low drop regulator. The value of the output voltage Vdc is set to be sufficiently large so that a maximum load current is supplied in the case where the load 108 is a maximum load. A difference between the output voltages Vdc and Vo is a loss LS100, which results in a reduction of efficiency of the power supply control device 100.